Semiconductor device, hemt device, and method of manufacturing semiconductor device

ABSTRACT

Provided is a semiconductor device in which a reverse leakage current is suppressed and the mobility of a two-dimensional electron gas is high. A semiconductor device includes: an epitaxial substrate in which a group of group-III nitride layers are laminated on a base substrate such that a (0001) crystal plane is substantially in parallel with a substrate surface; and a Schottky electrode. The epitaxial substrate includes: a channel layer made of a first group-III nitride having a composition of In x1 Al y1 Ga z1 N (x1+y1+z1=1, z1&gt;0); a barrier layer made of a second group-III nitride having a composition of In x2 Al y2 N (x2+y2=1, x2&gt;0, y2&gt;0); an intermediate layer made of GaN adjacent to the barrier layer; and a cap layer made of AlN and adjacent to the intermediate layer. A Schottky electrode is bonded to the cap layer.

TECHNICAL FIELD

The present invention relates to a semiconductor device, andparticularly to a semiconductor device in which a Schottky diodejunction is established between an epitaxial substrate having amultilayer structure made of a group-III nitride semiconductor and ametal electrode.

BACKGROUND ART

A group-III nitride semiconductor is attracting attention as asemiconductor material for a next-generation high-frequency/high-powerdevice, because the nitride semiconductor has a high breakdown electricfield and a high saturation electron velocity. For example, an HEMT(high electron mobility transistor) device in which a barrier layer madeof AlGaN and a channel layer made of GaN are laminated takes advantageof the feature that causes a high-concentration two-dimensional electrongas (2DEG) to occur in a lamination interface (hetero interface) due tothe large polarization effect (a spontaneous polarization effect and apiezo polarization effect) specific to a nitride material (for example,see Non-Patent Document 1).

In some cases, a single crystal (a different kind single crystal) havinga composition different from that of a group-III nitride, such assilicon and SiC, is used as a base substrate of an HEMT-devicesubstrate. In this case, a buffer layer such as a strained-superlatticelayer or a low-temperature growth buffer layer is generally formed as aninitially-grown layer on the base substrate. Accordingly, aconfiguration in which a barrier layer, a channel layer, and a bufferlayer are epitaxially formed on a base substrate is the most basicconfiguration of the HEMT-device substrate including a base substratemade of a different kind single crystal. Additionally, a spacer layerhaving a thickness of about 1 nm may be sometimes provided between thebarrier layer and the channel layer, for the purpose of facilitating aspatial confinement of the two-dimensional electron gas. The spacerlayer is made of, for example, AlN. Moreover, a cap layer made of, forexample, an n-type GaN layer or a superlattice layer may be sometimesformed on the barrier layer, for the purpose of controlling the energylevel at the most superficial surface of the HEMT-device substrate andimproving contact characteristics of contact with an electrode.

For example, it is known that, in a case where a nitride HEMT device hasthe most general configuration in which a channel layer is made of GaNand a barrier layer is made of AlGaN, the concentration of atwo-dimensional electron gas existing in an HEMT-device substrateincreases as the AlN mole fraction in AlGaN of the barrier layerincreases (for example, see Non-Patent Document 2). If the concentrationof the two-dimensional electron gas can be considerably increased, thecontrollable current density of the HEMT device, that is, the powerdensity that can be handled, would be considerably improved.

Also attracting attention is an HEMT device having a structure withreduced strain, such as an HEMT device in which a channel layer is madeof GaN and a barrier layer is made of InAlN, in which the dependence ona piezo polarization effect is small and almost only a spontaneouspolarization is used to generate a two-dimensional electron gas with ahigh concentration (for example, see Non-Patent Document 3).

In a case of preparing an HEMT device including a channel layer made ofGaN and a barrier layer made of InAlN, a junction formed between a gateelectrode and the barrier layer is generally a Schottky junction. Inthis case, however, depending on the composition of the InAlN layer andthe conditions under which the InAlN layer has been formed, there is apossibility that a large leakage current occurs when a reverse voltageis applied to the Schottky junction.

Forming a contact layer made of AlN on the InAlN layer can reduce theleakage current. However, an HEMT device having such a configurationinvolves a problem that the mobility of a two-dimensional electron gasis low. The cause thereof is assumed to be occurrence of strain in theInAlN layer due to the lattice constant of the AlN layer being smallerthan that of the InAlN layer.

PRIOR-ART DOCUMENTS Non-Patent Documents

-   Non-Patent Document 1: “Highly Reliable 250W High Electron Mobility    Transistor Power Amplifier”, TOSHIHIDE KIKKAWA, Jpn. J. Appl. Phys.    44, (2005), 4896-   Non-Patent Document 2: “Gallium Nitride Based High Power    Heterojunction Field Effect Transistors: process Development and    Present Status at USCB”, Stacia Keller, Yi-Feng Wu, Giacinta Parish,    Naiqian Ziang, Jane J. Xu, Bernd P. Keller, Steven P. DenBaars, and    Umesh K. Mishra, IEEE Trans. Electron Devices 48, (2001), 552

Non-Patent Document 3: “Can InAlN/GaN be an alternative to highpower/high temperature AlGaN/GaN devices?”, F. Medjdoub, J.-F. Carlin,M. Gonschorek, E. Feltin, M. A. Py, D. Ducatteau, C. Gaquiere, N.Grandjean, and E. Kohn, IEEE IEDM Tech. Digest in IEEE IEDM 2006, 673

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems describedabove, and an object of the present invention is to provide asemiconductor device in which a reverse leakage current is suppressedand the mobility of a two-dimensional electron gas is high.

To solve the problems described above, a first aspect of the presentinvention is a semiconductor device including: an epitaxial substrate inwhich a group of group-III nitride layers are laminated on a basesubstrate such that a (0001) crystal plane is substantially in parallelwith a substrate surface; and a Schottky electrode. The epitaxialsubstrate includes: a channel layer made of a first group-III nitridehaving a composition of In_(x1)Al_(y1)Ga_(z1)N (x1+y1+z1=1, z1>0); abarrier layer made of a second group-III nitride having a composition ofIn_(x2)Al_(y2)N (x2+y2=1, x2>0, y2>0); an intermediate layer made of GaNand adjacent to the barrier layer; and a cap layer made of AlN andadjacent to the intermediate layer. The Schottky electrode is bonded tothe cap layer.

A second aspect of the present invention is the semiconductor deviceaccording to the first aspect, wherein the intermediate layer has a filmthickness of 0.5 nm or more.

A third aspect of the present invention is the semiconductor deviceaccording to the second aspect, wherein the intermediate layer has afilm thickness of 6 nm or less.

A fourth aspect of the present invention is the semiconductor deviceaccording to any one of the first to third aspects, wherein the caplayer has a film thickness of 0.5 nm or more and 6 nm or less.

A fifth aspect of the present invention is the semiconductor deviceaccording to the first aspect, wherein the band gap of the secondgroup-III nitride is larger than the band gap of the first group-IIInitride.

A sixth aspect of the present invention is the semiconductor deviceaccording to the first aspect, wherein the Schottky electrode containsat least one of Ni, Pt, Pd, and Au.

A seventh aspect of the present invention is the semiconductor deviceaccording to the first aspect, wherein the cap layer has aroot-mean-square surface roughness of 0.5 nm or less.

An eighth aspect of the present invention is the semiconductor deviceaccording to the first aspect, wherein the second group-III nitride isIn_(x2)Al_(y2)N (x2+y2=1, 0.14≦x2≦0.24).

A ninth aspect of the present invention is the semiconductor deviceaccording to the first aspect, wherein the first group-III nitride isAl_(y1)Ga_(z1)N (y1+z1=1, z1>0).

A tenth aspect of the present invention is the semiconductor deviceaccording to the ninth aspect, wherein the first group-III nitride isGaN.

An eleventh aspect of the present invention is the semiconductor deviceaccording to the ninth aspect, further including a spacer layer providedbetween the channel layer and the barrier layer, the spacer layer beingmade of a third group-III nitride having a composition ofIn_(x3)Al_(y3)Ga_(z3)N (x3+y3+z3=1, y3>0) and having a band gap largerthan that of the second group-III nitride.

A twelfth aspect of the present invention is the semiconductor deviceaccording to the eleventh aspect, wherein the third group-III nitride isAlN.

A thirteenth aspect of the present invention is the semiconductor deviceaccording to the first aspect, wherein an ohmic electrode as well as theSchottky electrode is bonded to the cap layer.

A fourteenth aspect of the present invention is the semiconductor deviceaccording to the thirteenth aspect, which is an HEMT device, includingthe Schottky electrode serving as a gate electrode, and the ohmicelectrodes serving as a source electrode and a drain electrode.

A fifteenth aspect of the present invention is a method of manufacturinga semiconductor device. The semiconductor device includes: an epitaxialsubstrate in which a group of group-III nitride layers are laminated ona base substrate such that a (0001) crystal plane is substantially inparallel with a substrate surface; and a Schottky electrode. The methodincludes: a channel layer formation step of forming a channel layer on abase substrate, the channel layer being made of a first group-IIInitride having a composition of In_(x1)Al_(y1)Ga_(z1)N (x1+y1+z1=1,z1>0); a barrier layer formation step of forming a barrier layer on thechannel layer, the barrier layer being made of a second group-IIInitride having a composition of In_(x2)Al_(y2)N (x2+y2=1, x2>0, y2>0);an intermediate layer formation step of forming an intermediate layer,the intermediate layer being made of GaN and adjacent to the barrierlayer; a cap layer formation step of forming a cap layer, the cap layerbeing made of AlN and adjacent to the intermediate layer; and a Schottkyelectrode formation step of forming a Schottky electrode, the Schottkyelectrode being bonded to the cap layer.

A sixteenth aspect of the present invention is the method ofmanufacturing the semiconductor device according to the fifteenthaspect, wherein the intermediate layer is formed with a thickness of 0.5nm or more.

A seventeenth aspect of the present invention is the method ofmanufacturing the semiconductor device according to the sixteenthaspect, wherein the intermediate layer is formed with a thickness of 6nm or less.

An eighteenth aspect of the present invention is the method ofmanufacturing the semiconductor device according to the fifteenthaspect, wherein the cap layer is formed with a thickness of 0.5 nm ormore and 6 nm or less.

A nineteenth aspect of the present invention is the method ofmanufacturing the semiconductor device according to the fifteenthaspect, wherein the band gap of the second group-III nitride is largerthan the band gap of the first group-III nitride.

A twentieth aspect of the present invention is the method ofmanufacturing the semiconductor device according to the fifteenthaspect, wherein, in the Schottky electrode formation step, the Schottkyelectrode is formed so as to contain at least one of Ni, Pt, Pd, and Au.

A twenty-first aspect of the present invention is the method ofmanufacturing the semiconductor device according to the fifteenthaspect, wherein the second group-III nitride is In_(x2)Al_(y2)N(x2+y2=1, 0.14≦x2≦0.24).

A twenty-second aspect of the present invention is the method ofmanufacturing the semiconductor device according to the fifteenthaspect, wherein the first group-III nitride is Al_(y1)Ga_(z1)N (y1+z1=1,z1>0).

A twenty-third aspect of the present invention is the method ofmanufacturing the semiconductor device according to the twenty-secondaspect, wherein the first group-III nitride is GaN.

A twenty-fourth aspect of the present invention is the method ofmanufacturing the semiconductor device according to the twenty-secondaspect, further including a spacer layer formation step of forming aspacer layer between the channel layer and the barrier layer, the spacerlayer being made of a third group-III nitride having a composition ofIn_(x3)Al_(y3)Ga_(z3)N (x3+y3+z3=1, y3>0) and having a band gap largerthan that of the second group-III nitride.

A twenty-fifth aspect of the present invention is the method ofmanufacturing the semiconductor device according to the twenty-fourthaspect, wherein the third group-III nitride is AlN.

A twenty-sixth aspect of the present invention is the method ofmanufacturing the semiconductor device according to the fifteenthaspect, further including an ohmic electrode formation step of formingan ohmic electrode such that the ohmic electrode is bonded to the caplayer on which the Schottky electrode is formed.

In the first to twenty-sixth aspects of the present invention, theintermediate layer made of GaN and the cap layer made of AlN areprovided on the barrier layer in the mentioned order, and an electrodeis formed on the cap layer with a Schottky junction, to form an MISjunction. This achieves a semiconductor device in which a reverseleakage current is suppressed and the mobility of a two-dimensionalelectron gas is kept high as compared with a case where an electrode isformed directly on a barrier layer with a Schottky junction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A schematic cross-sectional view showing an outline configurationof an HEMT device 20 that is one aspect of a semiconductor deviceaccording to an embodiment of the present invention.

FIG. 2 A diagram illustrating the relationship between the surfaceroughness and thickness of a cap layer 6 b.

FIG. 3 A diagram illustrating the relationship between a reverse leakagecurrent and the cap layer 6 b.

FIG. 4 A diagram plotting a contact resistance of an ohmic electrodeagainst the thickness of the cap layer 6 b.

EMBODIMENT FOR CARRYING OUT THE INVENTION Configuration of HEMT Device

FIG. 1 is a schematic cross-sectional view showing an outlineconfiguration of an HEMT device 20 that is one aspect of a semiconductordevice according to an embodiment of the present invention. Roughly, theHEMT device 20 has a configuration in which a source electrode 7, adrain electrode 8, and a gate electrode 9 are arranged on an epitaxialsubstrate 10. More specifically, the epitaxial substrate 10 has aconfiguration in which a base substrate 1, a buffer layer 2, a channellayer 3, a spacer layer 4, a barrier layer 5, an intermediate layer 6 a,and a cap layer 6 b are laminated. Additionally, the source electrode 7,the drain electrode 8, and the gate electrode 9 are formed on the caplayer 6 b. The thickness ratio among the layers shown in FIG. 1 does notreflect the actual ratio. In one preferred example, all of the bufferlayer 2, the channel layer 3, the spacer layer 4, the barrier layer 5,the intermediate layer 6 a, and the cap layer 6 b are epitaxially formedthrough a MOCVD process (Metal Organic Chemical Vapor Deposition)(details will be described later).

The following description is directed to a case where the MOCVD processis used for the formation of each layer. However, a method appropriatelyselected from other epitaxial growth processes including vapordeposition processes and liquid phase deposition processes such as MBE,HVPE, and LPE may be adopted, or different growth processes may beadopted in combination, as long as the method can form each of thelayers with good crystallinity.

No particular limitation is put on the base substrate 1, as long as thebase substrate 1 allows a nitride semiconductor layer with goodcrystallinity to be formed thereon. In one preferable example, a singlecrystal 6H—SiC substrate is used. However, a substrate made of sapphire,Si, GaAs, spinel, MgO, ZnO, ferrite, or the like, may be adopted.

The buffer layer 2 is a layer made of AlN, and formed with a thicknessof about several hundreds nm, for the purpose of obtaining good crystalquality of the channel layer 3, the spacer layer 4, the barrier layer 5,the intermediate layer 6 a, and the cap layer 6 b which will be formedon the buffer layer 2. In one preferable example, the buffer layer 2 isformed with a thickness of 200 nm.

The channel layer 3 is a layer made of a group-III nitride (firstgroup-III nitride) having a composition of In_(x1)Al_(y1)Ga_(z1)N(x1+y1+z1=1), and formed with a thickness of about several μm. Thechannel layer 3 is preferably made of a group-III nitride having acomposition of Al_(y1)Ga_(z1)N (y1+z1=1, z1>0), and more preferably madeof GaN.

The barrier layer 5 is a layer made of a group-III nitride (secondgroup-III nitride) having a composition of In_(x2)Al_(y2)N (x2+y2=1,x2>0, y2>0), and formed with a thickness of about several nm to severaltens of nm. Preferably, 0.14≦x2≦0.24 is satisfied. The value of x2 beingoutside this range is not preferable, because strain acting on thebarrier layer 5 exceeds ±0.5%, at which crystal strain starts toincrease its influence on the reliability of a Schottky junction.

The channel layer 3 and the barrier layer 5 are formed so as to satisfysuch composition ranges that a band gap of the second group-III nitrideconstituting the latter is greater than a band gap of the firstgroup-III nitride constituting the former.

The intermediate layer 6 a is a layer made of GaN. The cap layer 6 b isa layer made of AlN. Functions and effects exerted by providing of theseintermediate layer 6 a and the cap layer 6 b in the HEMT device 20 willbe described later.

Moreover, the spacer layer 4 is provided between the channel layer 3 andthe barrier layer 5. The spacer layer 4 is a layer made of a group-IIInitride (third group-III nitride) having a composition ofIn_(x3)Al_(y3)Ga_(z4)N (x3+y3+z3=1) and containing at least Al (thatsatisfies y3>0), and formed with a thickness in the range from 0.5 nm to1.5 nm.

In the epitaxial substrate 10 having such a layer configuration, atwo-dimensional electron gas region 3 e, in which a two-dimensionalelectron gas exists with a high concentration, is formed at an interfacebetween the channel layer 3 and the spacer layer 4 (more specifically,in a portion of the channel layer 3 near this interface).

Preferably, the spacer layer 4 and the barrier layer 5 are formed so asto satisfy such composition ranges that a band gap of the thirdgroup-III nitride constituting the former is equal to or greater thanthe band gap of the second group-III nitride constituting the latter. Insuch a case, an alloy scattering effect is suppressed, and theconcentration and the mobility of a two-dimensional electron gas areimproved. More preferably, the spacer layer 4 is made of AlN (x3=0,y3=1, z3=0). In this case, the spacer layer 4 is a binary compound of Aland N, which can increase the suppression of the alloy scattering effectas compared with a ternary compound containing Ga. This improves theconcentration and mobility of the two-dimensional electron gas. Thediscussion about these composition ranges does not exclude that thespacer layer 4 contains impurities.

However, it is not essential that the spacer layer 4 is provided in theepitaxial substrate 10. It may be also acceptable that the barrier layer5 is formed directly on the channel layer 3. In this case, thetwo-dimensional electron gas region 3 e is formed at an interfacebetween the channel layer 3 and the barrier layer 5.

Each of the source electrode 7 and the drain electrode 8 is a multilayermetal electrode in which each metal layer has a thickness of about morethan ten nm to one hundred and several tens of nm. Each of the sourceelectrode 7 and the drain electrode 8 has an ohmic contact with the caplayer 6 b. The source electrode 7 and the drain electrode 8 may be madeof a metal material that provides a good ohmic contact relative to theepitaxial substrate 10 (relative to the cap layer 6 b). It is preferablethat multilayer metal electrodes made of Ti/Al/Ni/Au are formed as thesource electrode 7 and the drain electrode 8. However, this is notlimiting. For example, a multilayer metal electrode made of Ti/Al/Pt/Auor Ti/Al may be formed. The formation of the source electrode 7 and thedrain electrode 8 can be implemented through a photolithography processand a vacuum vapor deposition process.

On the other hand, the gate electrode 9 is a single-layer or multilayermetal electrode in which one or more metal layers are formed with athickness of about more than ten nm to one hundred and several tens ofnm. The gate electrode 9 has a Schottky contact with the barrier layer5. It is preferable that the gate electrode 9 is formed by using, as amaterial thereof, a metal having a high work function, such as Pd, Pt,Ni, or Au. Alternatively, it may be possible that a multilayer metalfilm consisted of some of the above-mentioned metals or consisted ofsome of the above-mentioned metals and Al, for example, is formed as thegate electrode 9. Since the cap layer 6 b made of AlN is provided, notonly the above-mentioned materials but also a metal material that isused for forming an ohmic junction with a group-III nitridesemiconductor is also adoptable as a material of the gate electrode 9.For example, a multilayer metal film containing Ti/Al may be formed.This is because, in such a case, a junction is made between AlN having awide band gap and a metal material having a relatively low workfunction, which allows a Schottky contact to be relatively easilyobtained. The formation of the gate electrode 9 can be implementedthrough a photolithography process and a vacuum vapor depositionprocess.

<Schottky Junction Between Cap Layer and Gate Electrode>

In the HEMT device 20 having the above-described configuration, the gateelectrode 9, the cap layer 6 b, and the barrier layer 5 (more exactly,with interposition of the intermediate layer 6 a) form a so-called MIS(metal-insulator-semiconductor) junction. Since the HEMT device 20 hassuch an MIS junction, in principle, a reverse leakage current issuppressed as compared with a conventional HEMT device in which the gateelectrode 9 has a Schottky junction directly with the barrier layer 5. Aspecific value varies depending on, for example, the composition and thethickness of each part. In a case where the HEMT device 20 has theconfiguration described in this embodiment, for example, a leakagecurrent caused upon application of −100V is suppressed down to about1/100 to 1/1000 of a leakage current occurring in a case where a gateelectrode is formed directly on a barrier layer.

FIGS. 2 to 4 are diagrams for explaining an effect exerted by the HEMTdevice in which the cap layer 6 b is provided immediately below the gateelectrode 9, that is, an effect exerted by the HEMT device having theabove-described MIS junction. More specifically, FIG. 2 illustrates therelationship between the surface roughness and the thickness of the caplayer 6 b with respect to three types of HEMT devices whose barrierlayers 5 have three different levels of compositions, namely,In_(0.14)Al_(0.86)N, In_(0.18)Al_(0.82)N, and In_(0.24)Al_(0.76)N.However, for convenience of discussion, the intermediate layer 6 a isnot provided in these HEMT devices. FIG. 3 illustrates the relationshipbetween a reverse leakage current and the thickness of the cap layer 6 bwith respect to the same HEMT devices. FIG. 4 illustrates therelationship between a contact resistance and the thickness of the caplayer 6 b with respect to the same HEMT devices.

In FIGS. 2 and 3, the value is highest when the thickness of the caplayer 6 b is 0 nm (in other words, when the cap layer 6 b is notprovided), and the value sharply drops until the thickness of the caplayer 6 b reaches 0.5 nm, and when the thickness of the cap layer 6 b is0.5 nm or more, the value remains almost unchanged at a value (0.5 nm orless) smaller than the value obtained when the thickness of the caplayer 6 b is 0 nm. This means that forming the cap layer 6 b with athickness of 0.5 nm or more can improve the surface flatness thereof andproviding the gate electrode 9 on the cap layer 6 b having such anexcellent surface flatness can reduce the reverse leakage current. Thesurface of the cap layer 6 b is more flattened than the surface of thebarrier layer 5.

In FIG. 4, when the thickness of the cap layer 6 b is 6 nm or less, thecontact resistance is almost constant at a value equal to or less than1.0×10⁻⁵/Ωcm², while when the thickness of the cap layer 6 b exceeds 6nm, the contact resistance rapidly increases. This result indicates thatit is preferable that the cap layer 6 b has a thickness of 6 nm or lessfrom the viewpoint of keeping a contact resistance of an ohmic electrodeat a sufficiently low value.

From the above, it is preferable to form the cap layer 6 b with athickness of 0.5 nm or more and 6 nm or less.

<Relationship Between Intermediate Layer and Two-Dimensional ElectronGas Concentration>

The HEMT device 20 according to this embodiment includes theintermediate layer 6 a provided between the barrier layer 5 and the caplayer 6 b. This is for the purpose of keeping a high mobility of thetwo-dimensional electron gas. More specifically, providing theabove-described cap layer 6 b directly on the barrier layer 5 reducesthe mobility of the two-dimensional electron gas, and therefore, in thisembodiment, to suppress such reduction, the intermediate layer 6 a isformed on the barrier layer 5 and then the cap layer 6 b is formedthereon.

The thickness of the intermediate layer 6 a is preferably 0.5 nm or moreand 6 nm or less. Forming the intermediate layer 6 a with a thickness of0.5 nm or more achieves a higher mobility as compared with when theintermediate layer 6 a is not provided. The upper limit of the thicknessof the intermediate layer 6 a may be set within a range where a sheetresistance is kept so low that no influence is given on it. For example,in a case where the thickness of the cap layer 6 b is 0.5 nm or more and6 nm or less, forming the intermediate layer 6 a with a thickness of(0.5 nm or more and) 6 nm or less can reduce the sheet resistance to300Ω/□ or less.

The HEMT device 20 according to this embodiment is also characterized inthat the intermediate layer 6 a and the cap layer 6 b are formed overthe whole surface of the barrier layer 5 so that they are uniformlyprovided not only immediately below the gate electrode 9 but alsoimmediately below the source electrode 7 and the drain electrode 8. Infact, the function and effect of reduction in the reverse leakagecurrent are obtained as long as the intermediate layer 6 a and the caplayer 6 b exist only immediately below the gate electrode 9, but toachieve such a configuration, a photolithography process, an etchingprocess, and the like, are required, which may cause a cost increase.This embodiment forms the intermediate layer 6 a and the cap layer 6 bover the whole surface of the barrier layer 5, without those processes.Therefore, it can be also said that an HEMT device having excellentcharacteristics is achieved with suppression of the costs. Needless tosay, it may be also possible that, in order to form the source electrode7 and the drain electrode 8 directly on the barrier layer 5, a so-calledrecessed ohmic in which the cap layer 6 b, the intermediate layer 6 a,and the barrier layer 5 are partially removed by etching is performedprior to forming those electrodes, and then the source electrode 7 andthe drain electrode 8 are formed on the barrier layer 5 exposed as aresult of the etching.

<Method for Preparing HEMT Device>

Next, a method for preparing the HEMT device 20 having theabove-described configuration will be described.

The preparation of the epitaxial substrate 10 can be performed by usinga known MOCVD apparatus. More specifically, an MOCVD apparatus is usedthat is configured to feed, into a reactor, a metal organic (MO) sourcegas (TMI, TMA and TMG) for In, Al and Ga, an ammonia gas (NH₃ gas), ahydrogen gas, and a nitrogen gas.

Firstly, for example, a (0001)-oriented 6H—SiC substrate having adiameter of two inches is prepared as the base substrate 1, and thisbase substrate 1 is placed on a susceptor provided in a reactor of theMOCVD apparatus. The inside of the reactor is vacuumed, and then, anatmosphere in a hydrogen/nitrogen mixed flow state is created while apressure inside the reactor is kept at a predetermined value in therange from 5 kPa to 50 kPa. In this condition, the susceptor is heatedto thereby raise the temperature of the substrate.

When the temperature of the susceptor reaches a predeterminedtemperature in the range from 950° C. to 1250° C. (for example, 1050°C.), which is a buffer layer formation temperature, an Al source gas anda NH₃ gas are introduced into the reactor, and thereby an AlN layerserving as the buffer layer 2 is formed.

After the formation of the AlN layer, the temperature of the susceptoris kept at a predetermined channel layer formation temperature, and ametal organic source gas and an ammonia gas are introduced into thereactor in accordance with the composition of the channel layer 3, andthereby an In_(x1)Al_(y1)Ga_(z1)N layer (x1=0, 0≦y1≦0.3) serving as thechannel layer 3 is formed. Here, the channel layer formation temperatureT1 is a value determined in a temperature range of 950° C. or more and1250° C. or less in accordance with a value of an AlN mole fraction y1in the channel layer 3. No particular limitation is put on the pressurein the reactor at a time when the channel layer 3 is formed. A pressurecan be appropriately selected from the range from 10 kPa to anatmospheric pressure (100 kPa).

After the formation of the In_(x1)Al_(y1)Ga_(z1)N layer, a nitrogen gasatmosphere inside the reactor is maintained while the temperature of thesusceptor is kept. The pressure in the reactor is set to be 10 kPa, andthen a metal organic source gas and an ammonia gas are introduced intothe reactor, so that an In_(x3)Al_(y3)Ga_(z3)N layer serving as thespacer layer 4 is formed with a predetermined thickness.

After the formation of the In_(x3)Al_(y3)Ga_(z3)N layer, then, in orderto form an In_(x2)Al_(y2)N serving as the barrier layer 5, thetemperature of the susceptor is kept at a predetermined barrier layerformation temperature that is 650° C. or more and 800° C. or less, andthe pressure in the reactor is kept at a predetermined value in therange from 1 kPa to 30 kPa. Then, an ammonia gas and a metal organicsource gas with a flow ratio in accordance with the composition of thebarrier layer 5 are introduced into the reactor such that the so-calledV/III ratio has a predetermined value of 3000 or more and 20000 or less.

After the formation of the In_(x3)Al_(y3)Ga_(z3)N layer, then thetemperature of the susceptor is set to be a predetermined intermediatelayer formation temperature. In this condition, a TMG and a NH₃ gas arefed, so that a GaN layer serving as the intermediate layer 6 a is formedwith a predetermined thickness.

After the formation of the GaN layer, then the temperature of thesusceptor is set to be a predetermined cap layer formation temperature.In this condition, a TMA and a NH₃ gas are fed, so that an AlN layerserving as the cap layer 6 b is formed with a predetermined thickness.Upon the formation of the cap layer 6 b, the preparation of theepitaxial substrate 10 is completed.

After the formation of the epitaxial substrate 10, this is used toprepare an HEMT device. Subsequent steps are achieved through a knownmethod.

Firstly, through a photolithography process and a vacuum vapordeposition process, multilayer metal patterns serving as the sourceelectrode 7 and the drain electrode 8 are formed in expected formationpositions on the cap layer 6 b.

Then, in order to give excellent ohmic characteristics to the sourceelectrode 7 and the drain electrode 8, the epitaxial substrate 10 havingthese source electrode 7 and drain electrode 8 formed thereon issubjected to a heat treatment for several tens of seconds in a nitrogengas atmosphere at a predetermined temperature of 650° C. to 1000° C.

Then, through a photolithography process and a vacuum vapor depositionprocess, a multilayer metal pattern serving as the gate electrode 9 isformed in an expected formation position on the cap layer 6 b.

Then, a resultant is singulated into chips each having a predeterminedsize. Thereby, a large number of HEMT devices 20 are obtained. On theHEMT device 20 thus obtained, die bonding and wire bonding are performedas appropriate.

As thus far described, in this embodiment, an intermediate layer made ofGaN is provided on a barrier layer, and a cap layer made of AlN isadditionally provided, and a gate electrode is formed on the cap layerwith a Schottky junction, to form an MIS junction. This achieves an HEMTdevice in which a reverse leakage current is largely reduced and themobility of the two-dimensional electron gas is high as compared with acase where a gate electrode is formed directly on a barrier layer with aSchottky junction.

<Modification>

Although the description of the above-described embodiment has beengiven with respect to an HEMT device, the aspect of forming an MISjunction between a gate electrode and a barrier layer is also applicableto other electronic devices using a Schottky junction, such as aSchottky barrier diode and a photosensor.

In the above-described embodiment, the cap layer 6 b is made of AlN.However, the cap layer 6 b may be made of a group-III nitride havinginsulating properties and having a band gap larger than that of thesecond group-III nitride. Here, a group-III nitride having insulatingproperties means a group-III nitride whose specific resistance is 10⁸Ωcm or more. When the specific resistance is in this range, theabove-described MIS junction is successfully formed. As long as thespecific resistance satisfies this range, the presence of conductiveimpurities in the cap layer 6 b is allowed.

EXAMPLES Example 1, Comparative Example 1, and Comparative Example 2

In an example 1, the epitaxial substrate 10 according to theabove-described embodiment including the intermediate layer 6 a and thecap layer 6 b was prepared, and evaluated for the two-dimensionalelectron gas concentration, the mobility of the two-dimensional electrongas, and the sheet resistance. Then, by using this epitaxial substrate10, four types of HEMT devices 20 were prepared which were differentfrom one another in terms of the configuration of the gate electrode 9.Each of the HEMT devices 20 was evaluated for a reverse leakage currentcaused upon application of −100V.

In a comparative example 1, an epitaxial substrate including neither theintermediate layer 6 a nor the cap layer 6 b was prepared, and evaluatedfor the two-dimensional electron gas concentration, the mobility of thetwo-dimensional electron gas, and the sheet resistance. Then, similarlyto the example 1, the gate electrode 9 was formed on this epitaxialsubstrate, to thereby prepare four types of HEMT devices. Each of theHEMT devices was evaluated for a reverse leakage current caused uponapplication of −100V.

Furthermore, in a comparative example 2, an epitaxial substrate notincluding the intermediate layer 6 a and including only the cap layer 6b was prepared, and evaluated for the two-dimensional electron gasconcentration, the mobility of the two-dimensional electron gas, and thesheet resistance. Then, similarly to the example 1, the gate electrode 9was formed on the epitaxial substrate, to thereby prepare four types ofHEMT devices. Each of the HEMT devices was evaluated for a reverseleakage current caused upon application of −100V.

That is, four types of gate electrodes 9 having different configurationswere formed on each of the three types of epitaxial substrates. Thus,twelve types of HEMT devices in total were obtained.

Firstly, the epitaxial substrate 10 was prepared. Until the formation ofthe spacer layer 4, all the epitaxial substrates 10 were prepared underthe same conditions.

To be specific, firstly, a plurality of (0001)-oriented 6H—SiCsubstrates having a diameter of two inches were prepared as the basesubstrate 1. The thickness thereof was 300 μm. Each of the substrateswas placed in a reactor of an MOCVD apparatus, and the inside of thereactor was vacuumed. Then, the pressure in the reactor was set to be 30kPa, and an atmosphere in a hydrogen/nitrogen mixed flow state wascreated. Then, the susceptor was heated, to thereby raise thetemperature of the base substrate 1.

After the temperature of the susceptor reached 1050° C., a TMA bubblinggas and an ammonia gas were introduced into the reactor, and an AlNlayer having a thickness of 200 nm was formed as the buffer layer.

Then, the temperature of the susceptor was set to be a predeterminedtemperature, a TMG bubbling gas serving as the metal organic source gasand an ammonia gas were introduced into the reactor with a predeterminedflow ratio. Thus, a GaN layer serving as the channel layer 3 was formedwith a thickness of 2 μm.

After the formation of the channel layer 3, the pressure in the reactorwas set to be 10 kPa, and then a TMA bubbling gas and an ammonia gaswere introduced into the reactor. Thus, an AlN layer having a thicknessof 1 nm was formed as the spacer layer 4.

After the formation of the spacer layer 4, the barrier layer 5 was thenformed with a thickness of 15 nm. The composition of the barrier layer 5was In_(0.18)Al_(0.82)N. The temperature of the susceptor was 745° C.

After the formation of the barrier layer 5, in the example 1, thetemperature of the susceptor was kept at 745° C. which is the barrierlayer formation temperature, and in this condition a GaN layer servingas the intermediate layer 6 a was formed with a thickness of 3 nm, andthen an AlN layer serving as the cap layer 6 b was formed with athickness of 3 nm. In the comparative example 2, the cap layer 6 b wasformed with a thickness of 3 nm. In the comparative example 1, nothingwas formed.

In each of the epitaxial substrates, after the final layer was formed,the temperature of the susceptor was lowered to the vicinity of a roomtemperature, and the inside of the reactor was returned to theatmospheric pressure. Then, the prepared epitaxial substrates 10 weretaken out. Through the above-described procedures, each of the epitaxialsubstrates 10 was obtained.

Then, a part of each of the epitaxial substrates was cut out in asingulating manner. A specimen to be evaluated thus obtained wassubjected to Hall effect measurement. In this measurement, thetwo-dimensional electron gas concentration, the two-dimensional electrongas mobility, and the sheet resistance was obtained with respect to eachof the epitaxial substrates.

Then, through a photolithography process and a vacuum vapor depositionprocess, an electrode pattern made of Ti/Al/Ni/Au (with film thicknessesof 25/75/15/100 nm, respectively) was formed on an upper surface of eachepitaxial substrate in expected formation positions where the sourceelectrode 7 and the drain electrode 8 were to be formed. Then, a heattreatment was performed in nitrogen for 30 seconds at 800° C.

Then, through a photolithography process and a vacuum vapor depositionprocess, a pattern of the gate electrode 9 was formed on the uppersurface of each epitaxial substrate in an expected formation positionwhere the gate electrode 9 was to be formed. Here, four types of gateelectrodes 9 in total were formed, namely, a single layer metalelectrode (12 nm) made of Au only and three types of multilayer metalelectrodes made of Ni/Au (with film thicknesses of 6 nm/12 nm), Pd/Au(with film thicknesses of 6 nm/12 nm), and Pt/Au (with film thicknessesof 6 nm/12 nm). The gate electrode 9 was formed such that the gatelength was 1 μm, the gate width was 100 μm, the interval between thegate electrode 9 and the source electrode 7 was 2 μm, and the intervalbetween the gate electrode 9 and the drain electrode 8 was 10 μm.

Finally, a resultant was singulated into chips. Thus, an HEMT device wasobtained.

Die bonding and wire bonding were performed on the obtained HEMT device,and then a reverse leakage current caused upon application of −100V wasmeasured.

Table 1 shows a list of, with respect to each of the HEMT devices, theconfigurations of the intermediate layer 6 a and the cap layer 6 b ofthe epitaxial substrate, the two-dimensional electron gas concentration,the mobility of the two-dimensional electron gas, the sheet resistance,the configuration of the gate electrode of the HEMT device, and a resultof measurement of the reverse leakage current caused upon application of−100V.

TABLE 1 Leakage Two-Dimensional Mobility of Current (A) Electron GasTwo-Dimensional Sheet (Upon Configuration of Concentration Electron GasResistance Configuration Application Epitaxial Substrate (/cm²) (cm²/Vs)(Ω/□) of Gate Metal of −100 V) Example 1 Intermediate 2.35E+13 980 271Ni/Au 1.46E−08 Layer: GaN Pd/Au 1.04E−08 (Thickness 3 nm) Pt/Au 4.62E−09Cap Layer: AlN Au 1.12E−08 (Thickness 3 nm) Comparative Intermediate2.30E+13 990 274 Ni/Au 2.88E−05 Example 1 Layer: Non Pd/Au 1.92E−05 CapLayer: Non Pt/Au 9.26E−06 Au 2.34E−05 Comparative Intermediate 2.50E+13650 384 Ni/Au 2.62E−08 Example 2 Layer: Non Pd/Au 2.21E−08 Cap Layer:AlN Pt/Au 1.12E−08 (Thickness 3 nm) Au 2.40E−08

The results shown in Table 1 reveal that, in all the HEMT devicesaccording to the example 1, that is, irrespective of the configurationof the gate electrode 9, the reverse leakage currents were suppressed toabout 1/100 to 1/1000 of the reverse leakage currents in the HEMTdevices according to the comparative example 1 in which the preparationwas performed under the same conditions except the intermediate layer 6a and the cap layer 6 b. It is also revealed that there is almost nodifference between the example 1 and the comparative example 1 in termsof the two-dimensional electron gas concentration, the mobility of thetwo-dimensional electron gas, and the sheet resistance.

On the other hand, in the HEMT devices according to the comparativeexample 2 in which the intermediate layer 6 a was not provided and onlythe cap layer 6 b was provided, the reverse leakage current wassuppressed to the same level as in the example 1, but the mobility ofthe two-dimensional electron gas was lower than in the example 1 and thecomparative example 1 while the sheet resistance was higher than in theexample 1 and the comparative example 1.

The above-described results indicate that providing the cap layer 6 bdirectly on the barrier layer 5 exerts an effect of reducing the leakagecurrent, but on the other hand, has a disadvantage in that the mobilityof the two-dimensional electron gas and the sheet resistance aredeteriorated, and also indicate that interposing the intermediate layer6 a between those layers can suppress deterioration in the sheetresistance, which is otherwise caused by reduction in the mobility ofthe two-dimensional electron gas, while maintaining the effect ofreducing the leakage current exerted by the cap layer 6 b.

In other words, providing the cap layer 6 b with the intermediate layer6 a being provided on the barrier layer 5 exerts an effect in terms ofreducing the reverse leakage current while maintaining a goodtwo-dimensional electron gas concentration and a good sheet resistance.

Example 2

In this example, HEMT devices whose intermediate layers 6 a haddifferent thicknesses, including a case of providing no intermediatelayer 6 a, were prepared. More specifically, the HEMT devices wereprepared through the same procedures as in the example 1, except thatthe thickness of the intermediate layer 6 a was varied in eight levelsof 0 nm, 0.1 nm, 0.5 nm, 1.5 nm, 3 nm, 6 nm, 8 nm, and 10 nm and thatthe material of the gate electrode 9 was only Ni/Au (with filmthicknesses of 6 nm/12 nm).

In the course of preparation of the HEMT devices, at a time when theepitaxial substrates were obtained, Hall effect measurement wasperformed similarly to the example 1. Thereby, the two-dimensionalelectron gas concentration, the mobility of the two-dimensional electrongas, and the sheet resistance were obtained with respect to each of theepitaxial substrates.

Additionally, similarly to the example 1, a reverse leakage current wasmeasured with respect to the obtained HEMT devices.

Table 2 shows a list of, with respect to each of the HEMT devices, thefilm thickness of the intermediate layer 6 a of the epitaxial substrate,the two-dimensional electron gas concentration, the mobility of thetwo-dimensional electron gas, the sheet resistance, and a result ofmeasurement of the reverse leakage current caused in the HEMT deviceupon application of −100V.

TABLE 2 Film Thickness Two-Dimensional Mobility of Leakage Current (A)of Intermediate Electron Gas Two-Dimensional Sheet Resistance (UponApplication of Layer (nm) Concentration (/cm²) Electron Gas (cm²/Vs)(Ω/□) −100 V) Example 2 0 2.50E+13 650 384 2.62E−08 0.1 2.48E+13 655 3842.18E−08 0.5 2.43E+13 975 263 1.86E−08 1.5 2.38E+13 982 267 1.42E−08 32.35E+13 980 271 1.46E−08 6 2.15E+13 988 294 1.36E−08 8 1.02E+13 988 6191.56E−08 10 8.50E+12 985 745 1.49E−08

As shown in Table 2, when the thickness of the intermediate layer 6 a is0.5 nm or more, the value of the mobility of the two-dimensionalelectron gas is higher than when the intermediate layer 6 a is notprovided. When the thickness of the intermediate layer 6 a is 6 nm orless, the value of the two-dimensional electron gas concentration is ata similar level to that obtained when the intermediate layer 6 a is notprovided. When the thickness of the intermediate layer 6 a is 0.5 nm ormore and 6 nm or less, the sheet resistance is kept at a value equal toor less than 300Ω/□, which is lower than when the intermediate layer 6 ais not provided.

On the other hand, as shown in Table 2, in any of the HEMT devices, thevalue of the leakage current is reduced to 1/1000 or less of thatobtained in the comparative example 1 (in the similar case where thegate electrode was Ni/Au) shown in Table 1.

From the above, it is revealed that forming the intermediate layer 6 awith a thickness of 0.5 nm or more between the cap layer 6 b and thebarrier layer 5 achieves an HEMT device in which a leakage current isreduced and the mobility of a two-dimensional electron gas is high. Itis further revealed that forming the intermediate layer 6 a with athickness of 6 nm or less achieves an HEMT device having a hightwo-dimensional electron gas concentration and a low sheet resistance.

1. A semiconductor device comprising: an epitaxial substrate in which agroup of group-III nitride layers are laminated on a base substrate suchthat a (0001) crystal plane is substantially in parallel with asubstrate surface; and a Schottky electrode, wherein said epitaxialsubstrate includes: a channel layer made of a first group-III nitridehaving a composition of In_(x1)Al_(y1)Ga_(z1)N (x1+y1+z1=1, z1>0); abarrier layer made of a second group-III nitride having a composition ofIn_(x2)Al_(y2)N (x2+y2=1, x2>0, y2>0); an intermediate layer made of GaNand adjacent to said barrier layer; and a cap layer made of AlN andadjacent to said intermediate layer, said Schottky electrode is bondedto said cap layer.
 2. The semiconductor device according to claim 1,wherein said intermediate layer has a film thickness of 0.5 nm or more.3. The semiconductor device according to claim 2, wherein saidintermediate layer has a film thickness of 6 nm or less.
 4. Thesemiconductor device according to claim 1, wherein said cap layer has afilm thickness of 0.5 nm or more and 6 nm or less.
 5. The semiconductordevice according to claim 1, wherein the band gap of said secondgroup-III nitride is larger than the band gap of said first group-IIInitride.
 6. The semiconductor device according to claim 1, wherein saidSchottky electrode contains at least one of Ni, Pt, Pd, and Au.
 7. Thesemiconductor device according to claim 1, wherein said cap layer has aroot-mean-square surface roughness of 0.5 nm or less.
 8. Thesemiconductor device according to claim 1, wherein said second group-IIInitride is In_(x2)Al_(y2)N (x2+y2=1, 0.14≦x2≦0.24).
 9. The semiconductordevice according to claim 1, wherein said first group-III nitride isAl_(y1)Ga_(z1)N (y1+z1=1, z1>0).
 10. The semiconductor device accordingto claim 9, wherein said first group-III nitride is GaN.
 11. Thesemiconductor device according to claim 9, further comprising a spacerlayer provided between said channel layer and said barrier layer, saidspacer layer being made of a third group-III nitride having acomposition of In_(x3)Al_(y3)Ga_(z3)N (x3+y3+z3=1, y3>0) and having aband gap larger than that of said second group-III nitride.
 12. Thesemiconductor device according to claim 11, wherein said third group-IIInitride is AlN.
 13. The semiconductor device according to claim 1,wherein an ohmic electrode as well as said Schottky electrode is bondedto said cap layer.
 14. The semiconductor device according to claim 13,which is an HEMT device, comprising said Schottky electrode serving as agate electrode, and said ohmic electrodes serving as a source electrodeand a drain electrode.
 15. A method of manufacturing a semiconductordevice, said semiconductor device comprising: an epitaxial substrate inwhich a group of group-III nitride layers are laminated on a basesubstrate such that a (0001) crystal plane is substantially in parallelwith a substrate surface; and a Schottky electrode, said methodcomprising: a channel layer formation step of forming a channel layer ona base substrate, said channel layer being made of a first group-IIInitride having a composition of In_(x1)Al_(y1)Ga_(z1)N (x1+y1+z1=1,z1>0); a barrier layer formation step of forming a barrier layer on saidchannel layer, said barrier layer being made of a second group-IIInitride having a composition of In_(x2)Al_(y2)N (x2+y2=1, x2>0, y2>0);an intermediate layer formation step of forming an intermediate layer,said intermediate layer being made of GaN and adjacent to said barrierlayer; a cap layer formation step of forming a cap layer, said cap layerbeing made of AlN and adjacent to said intermediate layer; and aSchottky electrode formation step of forming a Schottky electrode, saidSchottky electrode being bonded to said cap layer.
 16. The method ofmanufacturing the semiconductor device according to claim 15, whereinsaid intermediate layer is formed with a thickness of 0.5 nm or more.17. The method of manufacturing the semiconductor device according toclaim 16, wherein said intermediate layer is formed with a thickness of6 nm or less.
 18. The method of manufacturing the semiconductor deviceaccording to claim 15, wherein said cap layer is formed with a thicknessof 0.5 nm or more and 6 nm or less.
 19. The method of manufacturing thesemiconductor device according to claim 15, wherein the band gap of saidsecond group-III nitride is larger than the band gap of said firstgroup-III nitride.
 20. The method of manufacturing the semiconductordevice according to claim 15, wherein in said Schottky electrodeformation step, said Schottky electrode is formed so as to contain atleast one of Ni, Pt, Pd, and Au.
 21. The method of manufacturing thesemiconductor device according to claim 15, wherein said secondgroup-III nitride is In_(x2)Al_(y2)N (x2+y2=1, 0.14≦x2≦0.24).
 22. Themethod of manufacturing the semiconductor device according to claim 15,wherein said first group-III nitride is Al_(y1)Ga_(z1)N (y1+z1=1, z1>0).23. The method of manufacturing the semiconductor device according toclaim 22, wherein said first group-III nitride is GaN.
 24. The method ofmanufacturing the semiconductor device according to claim 22, furthercomprising a spacer layer formation step of forming a spacer layerbetween said channel layer and said barrier layer, said spacer layerbeing made of a third group-III nitride having a composition ofIn_(x3)Al_(y3)Ga_(z3)N (x3+y3+z3=1, y3>0) and having a band gap largerthan that of said second group-III nitride.
 25. The method ofmanufacturing the semiconductor device according to claim 24, whereinsaid third group-III nitride is AlN.
 26. The method of manufacturing thesemiconductor device according to claim 15, further comprising an ohmicelectrode formation step of forming an ohmic electrode such that saidohmic electrode is bonded to said cap layer on which said Schottkyelectrode is formed.